Abstract Due to the failure of monotherapies in T1D clinical trials it is widely thought that combination treatments that can control autoimmune responses and promote -cell health and replication will be necessary for T1D intervention. As a poof-of-concept, this proposal will test the immunoregulatory agent anti-CD3 with GABA, the latter of which has both immunoregulatory actions and is a -cell mitogen/survival factor. Previous clinical studies with anti-CD3 have failed to maintain normoglycemia in newly diabetic individuals, which may have been due to 1) the chronic exhaustion of the remaining -cells, 2) the lack of sufficient -cell regeneration and 3) insufficient suppression of autoimmunity. By combining anti-CD3 with GABA, each of these deficiencies can be improved, i.e., GABA promotes mouse and human -cell survival, -cell replication and mass, and inhibits inflammatory immune cells while enhancing Treg responses. This is unlike any previously tested combination with anti-CD3 (or other immunoregulatory agent) because the second therapeutic agent used in those studies lacked the ability to inhibit autoimmunity/promote Tregs or was not a -cell mitogen/survival factor. We hypothesize that the combination of anti-CD3+GABA will sufficiently control autoimmune responses such that GABA's mitogenic and anti-apoptotic actions will be able to better preserve, and perhaps even expand -cell mass, in newly diabetic NOD mice. Since no mono- or combined therapy has yet been able to enhance -cell replication and mass in diabetic NOD mice, a successful outcome would be novel and have high potential for clinical translation. Additionally, because the actions of anti-CD3 and GABA are expected to synergize, we will test whether lower dosages of each treatment can be effective, which could be useful in the clinic to reduce potential side-effects. Finally, in addition to monitoring the effect of this combination treatment on immune responses and -cell replication, mass and function, we will determine the levels of GABA in plasma that are associated with inhibition of autoreactivity and promotion of -cell cell replication, which will be informative for GABA dosing in clinical trials. The results have high clinical potential to better preserve and perhaps increase -cell mass after T1D onset. Even a small preservation or increase of -cell mass will have clinical benefits in those newly diagnosed with T1D by virtue of lowering insulin requirements, improving glucose control and thereby reducing the long-term risk for complications. Our approach is not limited to combining GABA with anti- CD3--in the future, GABA could be combined with other safe immunoregulatory drugs, so that a proof-of- concept is likely to lead to many new possibilities for T1D intervention.